The major effort to produce high Tc superconducting wires and tapes has been focused on the oxide powder in tube (OPIT) process. Furthermore, bismuth- and thallium-based superconductors have been the material of choice since platelike morphology can be obtained in both of these type superconductors after the OPIT process, thus reducing the known weak-link problem. Although tremendous progress has been made in recent years in producing long high T.sub.c wires and tapes by using OPIT, only limited success has been achieved in producing a uniform microstructure of the right superconducting phase over long lengths.
Very little effort has been focused on production of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. (YBCO) wires and tapes due to serious weak-link problems in the YBCO superconductor. Melt-textured processing has yielded excellent YBCO superconductors, but it is considered a difficult process to scale up for long lengths. A possible process has been referred to as the thick film process. In the thick film process, it has been shown that YBCO thin films on single crystal substrates have critical current density (J.sub.c) values of over 10.sup.6 amperes per square centimeter (A/cm.sup.2) at 77K. There have been a number of reports on depositing buffer layers on metal substrates which can be easily obtained in long lengths as compared to flexible polycrystalline yttria-stabilized zirconia (YSZ) substrates.
One process of depositing a YSZ layer has been by use of ion beam assisted deposition (IBAD) in which a YSZ layer is deposited in combination with irradiation from an ion beam directly on a substrate during the deposition. For example, both lijima et al., Appl. Phys. Lett., vol. 74, no. 3, pp. 1905 (1993) and Reade et al., Appl. Phys. Lett., vol. 61, no. 18, pp. 2231-2233 (1992) have demonstrated deposition of highly in-plane textured YSZ buffer layers with IBAD, leading to YBCO thin films having excellent superconducting properties. Another development has been the addition of a layer between the substrate surface and the YSZ layer. For example, Chatterjee et al., Physica C, v. 224, pp. 286-292 (1994) describe the use of metallic underlayers of platinum, palladium, gold or silver as a barrier layer between a metallic substrate and the YSZ layer.
Despite the results from previous work on developing YBCO superconducting films on flexible polycrystalline metal substrates, further improvements in the resultant properties have been desired. After extensive and careful investigation, improvements have been found to the preparation of YBCO superconducting films on polycrystalline substrates such as flexible polycrystalline metal substrates, each improvement contributing in portion to the resultant desired properties.
It is an object of the present invention to provide YBCO superconducting films on polycrystalline substrates demonstrating improved properties.